The barrier-free society is highly needed recently. From such viewpoint, in audio communication appliances, for the elderly people or others having difficulty in hearing, there is an increasing demand for development of technology for generating a more audible audio signal by expanding apparently the band of audio signal. Usually, the audio signal through telephone line is one of the standards in audio communication mainly by verbal expression. The audio signal by telephone is limited in the frequency band width, and its tone quality is not generally excellent as compared with the original voice. For example, even in the wire telephone line of relatively favorable tone quality, the actual audio frequency band width is limited to about 300 to 3500 Hz, witch is about half of human voice frequency range. The human vocal frequency range is generally composed of fundamental frequencies of 80 Hz to 800 Hz and higher harmonics of several degrees thereof in the Japanese male voice, and is 150 Hz to 1600 Hz and higher harmonics of several degrees thereof in the Japanese female voice. Including the higher harmonics of several degrees and further voiceless sound, the vocal range is as wide as 80 Hz to 16 kHz. It is important, in comprehension of spoken words and pleasant tone quality of spoken words particularly, for the human voice to have the frequency components including harmonics of several degrees higher than 3500 Hz and to have the frequency components including the fundamental frequencies of spoken words lower than 300 Hz. The telephone line and other transmission line of narrow frequency band dissipate not only the majority of these important high and low frequency components of voiced sound but also high frequency components of voiceless sound, so that they deteriorate the tone quality of verbal audio signal in hearing. Narrowed frequency band of audio signal impedes ease of hearing and the comprehension degree (intelligibility). As a result, narrowed frequency band of audio signal is reported to be a serious problem for the elderly people or others having difficulty in hearing (Proceeding of the 1996 Autumn Meeting of Acoustic Society of Japan, Vol.1, 2-6-5, 1996).
To solve such problems, various methods and devices for expanding the frequency band of audio signals have been attempted so far. However, although they require a tremendous quantity of operation steps and memory capacity, but the processed signals are not always satisfactory in the tone quality. For example, one of a typical conventional method is the codebook mapping method, which matches an input voice of telephone frequency band with a recorded voice of wide frequency band by using a codebook, so as to generate high quality voice. But, this method not only requires a lot of matching operation steps and a lot of memories for codebook and for recorded high quality voices, but also tends to be unstable in the matching precision depending on the line status.
Another expansion method of audio frequency is on the synthesis by analysis method, which also requires a detailed and tremendous operation steps for analysis and then for synthesis. The obtained result is not satisfactory as compared with the required cost for installation.
An exceptional method has been proposed to compensate virtually the high frequency signal by using aliasing signal and fixed filter in small operation steps. This method, however, cannot expand sufficiently the high frequency range of voiceless sound, and so it does not improve the clarity and perception of sound, and then results in a dull sound.